Almost half of U.S. lighting energy goes into commercial indoor
lighting. This sector includes stores, offices, and schools. This explains why
lamp makers have committed so much of their research resources to improving
fluorescent lamps--the most common lamp in the commercial sector.

While
residential lighting energy seems small in comparison (only 20%), this is a bit
misleading. Most homes and apartments are lighted by incandescent lamps that are
cheap and easy to use, but very inefficient. As there are so many residential
users of light, even small gains can mean big savings if enough people adopt the
more efficient product.

To read the chart above, a few definitions are needed.
Electricity consumption is measured in "watt-hours" which is simply
the number of watts used times the number of hours the electricity flows. A
common unit of measure for home owners is the "kilowatt-hour." One
kilowatt-hour is equal to 1000 watt-hours. Measuring electricity on a national
scale calls for even larger increments, though. "TWh" in the chart
stands for terawatt-hours. One TWh is equal to one billion kilowatt-hours.

Electricity
is not the only form of energy, however. A common measurement of energy is the
British Thermal Unit or BTU. A BTU is the quantity of heat needed to raise the
temperature of one pound of water one degree Fahrenheit. "Quad" in the
chart stands for one quadrillion BTUs, and is a measure often used in energy
industries.

Chart by Lee R. Anderson, compiled from information in "Analysis of Federal Policy Options for Improving U.S. Lighting Energy Efficiency: Commercial
and Residential Buildings," December 1992, Lawrence Berkeley National Laboratory, U.S. Department of Energy.